This invention pertains generally to absolute optical encoders and more particularly to an absolute optical encoder using a line array of detector elements.
In the past, absolute optical encoders have been devised typically by providing several strips of alternating dark and light bands; each band with a detector and each band having a resolution differing in count density typically by a factor of two relative to the adjacent strips. The coarsest strip is a dark zone covering half the measurement field and a light zone covering the other half. The highest density strip has enough alternating bands to provide the resolution required for a particular encoder function. Generally, the on-off sequence of detectors can be interpreted as a binary number defining the encoder position. The number of discernable positions on the encoder is 2.sup.n, where n is the number of strips of alternating bands. A refinement on the known art involves the use of pairs of transparent and opaque bars optically superimposed on the encoder pattern which are staggered relative to one another in the position at which the maximum transmission occurs by 1/4 of the spacing between adjacent dark bar centers. Such a refinement yields the ability to measure position to a fraction of the bar center spacing. Known optical encoders are generally used either for measuring position along a line of motion or angle of rotation around a fixed center. In the linear position mode, the known encoders must be constrained to limit orthogonal to strip motion to less than the distance separating adjacent strips of alternating dark and light encoder bands.
In the known designs for absolute optical encoders, relative motion between encoder bands and detectors is sensed by a variety of means. One means is to use transparent and opaque bands with collimated light passing through to detectors or slit apertures in close proximity to the encoder bars. A second approach involves imaging the encoder bars onto detectors or slit apertures. Each of the known encoder designs provides a means of superimposing a shadow or image of the encoder bars on the detector elements or on apertures passing light to the detector elements.